Field of the Invention
The present invention relates to an image forming apparatus which employs an electrophotographic system, such as a copying machine and an LBP, and has a fixing apparatus for heat fixing a toner image formed on a recording material.
Description of the Related Art
As a fixing apparatus provided in electrophotographic copying machines and printers, a device employing a heating roller and a device employing a film are known. In any fixing apparatus, when unfixed toner on a recording material partially adheres to the surface of a fixing roller or a film, and then a portion to which the toner adheres contacts a recording material next, an image defect referred to as “offset” in which the toner moves to the side of the recording material occurs in some cases. In order to suppress the offset, a configuration of increasing electrostatic adhesion to a recording material of toner is known. Japanese Patent Laid-Open No. 9-80946 discloses performing control so that toner is easily electrostatically held on a recording material by applying a voltage having the same polarity as that of the toner to a base material portion of a fixing film.
Known as a conspicuous image defect among some types of electrostatic offset is separating offset. The separating offset occurs due to remaining of a locally charged portion having a polarity opposite to the polarity of toner caused by strong separating discharge occurring between a fixing nip portion and the surface of a fixing film when a back end portion of the recording material passes through the fixing nip portion in the case where the recording material has a high resistance value.
Although the separating offset can be suppressed by applying a voltage in a direction in which a separating offset electric field is canceled, i.e., a voltage in a direction in which the electrostatic adhesion to a recording material of toner increases, the following conditions arise.
The conditions are as follows. When a large amount of paper dust containing calcium carbonate as the main component is contained in a recording material, the paper dust has a property of easily being positively charged. Therefore, when a voltage to be applied to a pressure roller is large, the positively charged paper dust is easily adsorbed to the side of a fixing film. The paper dust adsorbed to the side of the fixing film is gradually accumulated. When the accumulation amount increases, the release performance of fluororesin of the surface of the fixing film is reduced. When the release performance decreases, the amount of offset toner adhering to the film also increases, so that “toner fouling” in which the paper dust and the toner are mixed occurs. The toner fouling is sometimes accumulated on the surface of the fixing film, sometimes accumulated on the surface of the pressure roller, or sometimes accumulated on both the surface of the fixing film and the surface of the pressure roller. The accumulation manner is determined by the intrinsic characteristics of an apparatus depending on the balance of release performance between the fixing film and the pressure roller or the characteristics of materials in many cases. In any case, when the accumulation amount of the toner fouling exceeds a fixed amount, an image defect caused by discharge of the toner fouling as a toner residue to the surface of the recording material sometimes occurs.